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Plant Cells and Tissues
Plants became truly land-dwelling with the advent of:
1. spores with durable, protective walls
2. thickened waxy cuticle over the epidermis
3. stomates that open and close
4. our old pal, lignin:
5. progression of alternation of generations so
that the gametophyte is a small, ephemeral stage, and the sporophyte
a large, persistent stage. (More on this later)
Once plants invaded terrestrial habitats, natural selection took over, and individuals with adaptive mutations radiated and changed to pioneer an entirely new world available to them. And that meant changing from unicellular, relatively undifferentiated ancestral algae into complex organisms with specialized cell types as well as simple and complex tissues and organs.
PLANT ORGANS
There are only three:
Plant organs are generally defined by the presence of
more than one type of tissue. So before we embark on our study of plant
organs, let's have a look at their components.
PLANT CELLS
One picture is worth a lot of words.
PLANT TISSUES
A tissue is defined as an aggregation of cells coordinated to perform a
particular function or set of functions.
Tissues may be
- meristematic (embryonic and totipotent)
- simple (composed of only one type of cell), such as
- parenchyma
- collenchyma
- sclerenchyma
- complex (composed of more than one type of cell), such as
- dermal (protective covering)
- vascular (conducting tissue)
- xylem (conducts water and dissolved minerals)
- phloem (conducts water and dissolved organics)
- ground (bulk of the body; primarily parenchyma, collenchyma &
sclerenchyma)
MERISTEMS: Source of all other tissues
Meristems are regions of undifferentiated, embryonic
cells. Initially, the cells
are totipotent and can differentiate/mature into any other type
of cell.
Recall the terminology for sequentially less versatile undifferentiated cells:
- totipotent cells have the capacity to develop into any type of cell.
- pluripotent cells can develop into many, but not all different types of cells.
- multipotent cells can develop into multiple types of cells, but not as many types as pluripotent cells.
Meristems are present throughout the life of the plant, and are the source of seasonal new growth in both height (primary meristems) and girth (secondary meristems).
Meristematic cells are also found at the margins of such structures as growing leaves or flower petals. As they divide, they differentiate and lose their totipotency until the structure is fully grown and mature.
Height and Girth: Primary and Secondary Meristems
Primary meristems: located at the tips of roots
and shoots. Responsible for increase in plant length. Apical meristems are located in two different areas of the stems and roots:
- apical buds (at the tips roots and shoots)
- axillary buds (located in the leaf axils)
Secondary (a.k.a. lateral) meristems: located in the margins of the
stem and root (vascular and cork cambium). Responsible for increase
in girth.
- vascular cambium - gives rise to secondary xylem and phloem
- cork cambium - give rise to the periderm, which replaces the epidermis in woody plants.
Note: Only plants with a vascular cambium can produce true, botanical WOOD, which is composed of concentric rings of xylem.
Here are where you'll find the meristems...
Primary growth and primary structures arise from the apical/primary meristems.
Secondary growth and secondary structures arise from the lateral meristems.
CELLS OF THE GROUND TISSUES
- parenchyma - the generalized, box-shaped cell with uniformly thick cell walls
(when parenchyma cells have a high density of chloroplasts, they are sometimes
referred to as "chlorenchyma" cells)
- collenchyma - corners of the cell wall are thickened with cellulose
- sclerenchyma - highly lignified and cellulose-rich cell walls, these are dead and hollow at maturity.
- fibers - long, slender, tapered cells
- sclerids - small, granular clusters
CELLS OF THE VASCULAR TISSUES
- XYLEM
Xylem conducts water and inorganic substances from the roots to the aboveground parts of the plant, and out the stomates. A complex tissue, it consists of several different types of cells.
- conducting cells (tracheids and/or vessel elements)
- tracheids - long, tapering cells covered with multiple pits
- vessel elements - tubelike and of greater diameter than tracheids, these are stacked end-to-end to form long, strawlike structures called vessels.
At maturity, tracheids and vessel elements are dead and hollow, lacking a living protoplast.
Vessel elements, though they conduct water more rapidly, are more "risky" for the plant. Unlike tracheids, in which water must pass through the pit membranes, vessel elements have large pores. This means that air bubbles formed (e.g., during freezing and thawing of xylem sap in the spring) will be trapped inside an individual tracheid, and cannot block water flow.
A vessel element can accumulate bubbles, and one good-sized bubble can break the water column in an entire vessel, making it useless. (Think of your cut roses!)
The growing conditions under which tracheids and vessel elements form determines whether they will be either flexible or rigid at maturity. Once the conducting cell has reached its final size and shape, apoptosis (programmed cell death) takes place, and the protoplast disintegrates, leaving a hollow conducting element.
ground cells (parenchyma, collenchyma, sclerenchyma)
These form the structural support around the conducting cells.
- parenchyma - provide nutrient storage and storage of other substances, such as aromatic and other protective secondary metabolites. These are present in vertical strands in primary xylem, and also in radially arranged rays in secondary xylem.
Sclerenchyma fibers form a large part of wood biomass.
PHLOEM
More about companion cells and albuminous cells...
In angiosperms
each sieve tube member is associated
with a COMPANION CELL, which is a fully equipped parenchyma cell
that performs all necessary metabolic functions for both cells.
- The two are joined by numerous plasmodesmata, creating
a large surface area for transport between the two cell membranes.
- The two are derived from the same progenitor (mother) cell.
In gymnosperms
each sieve cell is associated with
an albuminous cell.
- This performs the same function as the companion
cell in angiosperms, but it is NOT derived from the same progenitor
cell as the sieve cell it serves.
DERMAL TISSUES
These are the "skin" of the plant. Epidermis is found in herbaceous plants, and in those that become woody, it is replaced by the periderm, which originates from the cork cambium.
EPIDERMIS
This very complex tissue consists largely of flattened cells
that lack chloroplasts. Specialized cells found in the epidermis
include
- guard cells (bordering the stomates)
- trichomes (cell with a long, hairlike extension)
The botanical terms...
- glabrous: hairless
- pubescent: fuzzy
- hirsute: hairy
...all refer to the presence or absence of foliar trichomes.
e.g. - root hairs, surface hairs on leaves & stems,
special absorbing trichomes in epiphyte foliage, salt-secreting
cells
- gland cells - produce and/or store a wide variety of compounds, from nectar to
poisons (often associated with trichomes) to...just about anything.
PERIDERM
This is a secondary epidermis, produced by the cork
cambium. It consists of
- phellum (cork) which is heavily impregnated
with waxy suberin.
- cork cambium
Directly beneath the periderm lies the phloem. The periderm and the phloem together comprise the BARK of a woody plant.
The overall arrangement of tissues in a generalized stem cross-section can be seen here:
And the progression of their development:
